1. Field of the Invention
The present invention relates to an apparatus and a method that enhance depth feel and stereoscopic effect of an image formed by an imaging apparatus, such as a movie camera or a digital still camera (DSC), by capturing an image of a target space and estimating a distance to an object within the target space.
2. Description of the Related Art
Three-dimensional space measuring techniques have potential applications in many fields. To put the 3D space measuring techniques into practical use, a variety of methods have been used. Typical methods include light sectioning of laser slit light using scanning, triangulation such as stereo triangulation, and a TOF method for determining a distance by illuminating a measurement-target object with illumination light and timing the time-of-flight (TOF), or the time required by the illumination light to return from the object.
The 3D space measurement using triangulation requires light scanning to be performed in a target space (a three-dimensional space whose image is to be captured) to obtain 3D information of the target space. The measurement using triangulation requires a relatively long time to obtain 3D information of the entire target space. Thus, triangulation is considered unsuitable for applications that would require, for example, tracking of a moving object.
Unlike triangulation, the 3D space measurement using the TOF method does not require such laser beam scanning. The 3D space measurement using the TOF method enables a distance to an object (distance from an imaging apparatus to an object) to be detected at a high speed in units of pixels of a television image (captured image), and also enables its distance measurement range to be set relatively large (set to about 3 meters or more). Moreover, the 3D space measurement using the TOF method does not need to use a laser light source but can use a light-emitting diode (LED), which enables safe imaging of humans. With these advantages of the TOF method, various approaches to perform the 3D space measurement using the TOF method have been reported, and commercial distance sensors that perform the 3D space measurement using the TOF method have also been reported.
With the TOF method, a distance in a three-dimensional space is determined using equation 1. The distance L, which is a distance between a measurement reference point and a measurement target point, is determined using the following equation.Equation 1L=c·Δt/2  (1)where c is the speed of light, which is written as c=3.0*10^8 [m/sec], and Δt is the time required from when light emitted from a light source, which is at the measurement reference point, illuminates a measurement target, which is at the measurement target point, to when its reflected light from the measurement target returns to the light source at the measurement reference point. In other words, Δt is the round-trip time of the light from the light source to the measurement target.
The TOF method may be implemented based on many different methods. Typical methods are a phase TOF method and a pulse TOF method.
With the phase TOF method, a measurement-target object is illuminated mainly with a light beam having a modulated intensity, reflected light from the measurement-target object is detected and converted through photoelectronic conversion, and photoelectrons resulting from the conversion are accumulated into a plurality of accumulation units in a temporally shifted manner, and distance information is generated according to the number of photoelectrons accumulated in these accumulation units.
With the pulse TOF method, a measurement-target object is illuminated with a pulsed light beam, and a distance is measured using a phase difference between reflected light from the measurement-target object and the measurement light beam. In detail, two-dimensional scanning is performed using the measurement beam, and a distance at each measurement point is measured to obtain a three-dimensional shape.
With the phase TOF method, a distance is obtained using a phase amount ΔΦ instead of the time Δt in equation 1. With the phase TOF method, a distance at which the phase amount ΔΦ is 2π (or at which Δt is T: the one-cycle time of the modulated intensity) is a maximum detected distance Lmax, which is determined using equation 2. More specifically, the maximum detected distance Lmax depends on a modulation frequency f of the measurement light beam, and is determined using the following equation.Equation 2Lmax=c/2f  (2)
The distance L is determined using equation 3 with the phase amount ΔΦ.Equation 3L=(Lmax×Δφ)/2π  (3)
However, when the distance to the measurement-target object is greater than or equal to the value of the wavelength corresponding to the cycle of the intensity modulation of the measurement light beam, the phase TOF method in principle would fail to obtain a uniquely determined distance calculation result (in other words, this method would fail to determine the distance to the measurement-target object).
To obtain a distance image, the pulse TOF method requires two-dimensional scanning using a measurement light beam. The pulse TOF method requires physical scanning of a measurement light beam, such as laser light emitted from a light source, from side to side and up to down by using an oscillating mirror or a polygon mirror. The pulse TOF method consequently requires a long time to obtain a distance image.
Many currently-available techniques of 3D space measurement use the phase TOF method or the like (see, for example, Patent Citations 1 and 2). Conventional distance estimation apparatuses (conventional examples 1 and 2) using the phase TOF method will now be described.